1. Field of the Invention
The present invention relates to a magnetoresistive device including a magnetoresistive element and a bias magnetic field applying layer, and to a thin-film magnetic head, a head gimbal assembly, a head arm assembly and a magnetic disk drive each including this magnetoresistive device.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write head having an induction-type electromagnetic transducer for writing and a read head having a magnetoresistive element (that may be hereinafter referred to as MR element) for reading are stacked on a substrate.
MR elements include giant magnetoresistive (GMR) elements utilizing a giant magnetoresistive effect, and tunneling magnetoresistive (TMR) elements utilizing a tunneling magnetoresistive effect.
Read heads are required to have characteristics of high sensitivity and high output capability. As the read heads that satisfy such requirements, GMR heads that employ spin-valve GMR elements have been mass-produced. Recently, to adapt to further improvements in areal recording density, developments have been pursued for read heads employing TMR elements.
Typically, a spin-valve GMR element includes: a nonmagnetic conductive layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the two surfaces of the nonmagnetic conductive layer; a pinned layer disposed adjacent to the other of the two surfaces of the nonmagnetic conductive layer; and an antiferromagnetic layer disposed adjacent to a surface of the pinned layer farther from the nonmagnetic conductive layer. The free layer is a ferromagnetic layer whose direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer whose direction of magnetization is fixed. The antiferromagnetic layer is a layer that fixes the direction of magnetization of the pinned layer by means of exchange coupling with the pinned layer.
Conventional GMR heads have a structure in which a current used for detecting magnetic signals (that is hereinafter called a sense current) is fed in the direction parallel to the plane of each layer making up the GMR element. Such a structure is called a current-in-plane (CIP) structure. On the other hand, developments have been made for another type of GMR heads having a structure in which the sense current is fed in a direction intersecting the plane of each layer making up the GMR element, such as the direction perpendicular to the plane of each layer making up the GMR element. Such a structure is called a current-perpendicular-to-plane (CPP) structure. A GMR element used for read heads having the CPP structure is hereinafter called a CPP-GMR element. A GMR element used for read heads having the CIP structure is hereinafter called a CIP-GMR element.
A read head that employs the TMR element mentioned previously is of the CPP structure. Typically, the TMR element includes: a tunnel barrier layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the two surfaces of the tunnel barrier layer; a pinned layer disposed adjacent to the other of the two surfaces of the tunnel barrier layer; and an antiferromagnetic layer disposed adjacent to a surface of the pinned layer farther from the tunnel barrier layer. The tunnel barrier layer is a nonmagnetic insulating layer that allows electrons to pass therethrough with spins thereof maintained by means of the tunnel effect. The free layer, the pinned layer and the antiferromagnetic layer are the same as those of the spin-valve GMR element.
Typically, bias magnetic field applying layers for applying a bias magnetic field to the free layer are provided on two sides of the MR element that are opposed to each other in the direction of track width. The bias magnetic field aligns the magnetization of the free layer to a certain direction in the absence of any signal magnetic field applied to the MR element, and thereby brings the free layer into a single magnetic domain state. Occurrence of Barkhausen noise in output signals of the read head is thereby suppressed. Each bias magnetic field applying layer is typically formed of a hard magnetic layer having a high coercivity or a stack of a ferromagnetic layer and an antiferromagnetic layer, for example.
FIG. 11 illustrates an example of a read head including bias magnetic field applying layers having a typical configuration. FIG. 11 is a cross-sectional view of the example of the read head. This read head has the CPP structure. The read head illustrated in FIG. 11 includes: a first shield layer 103, an MR element 105 disposed on the first shield layer 103, and a second shield layer 108 disposed on the MR element 105. The MR element 105 is a CPP-GMR element or a TMR element. This read head further includes two bias magnetic field applying layers 106 that are respectively disposed adjacent to two side surfaces of the MR element 105 and apply a bias magnetic field to the MR element 105, and an insulating layer 109 disposed between each bias magnetic field applying layer 106 and a stack of the first shield layer 103 and the MR element 105. The shield layers 103 and 108 also function as electrodes for feeding a sense current to the MR element 105. In this example, the bias magnetic field applying layers 106 are each formed of a single hard magnetic layer.
JP9-180134A, JP8-315325A, and JP8-050709A disclose bias magnetic field applying layers having a configuration other than the typical configuration described above. The bias magnetic field applying layers disclosed in JP9-180134A are each formed of a stack of three or more layers including a ferromagnetic layer, an antiferromagnetic layer and a ferromagnetic layer. In the bias magnetic field applying layers of this publication, the ferromagnetic layer and the antiferromagnetic layer are exchange-coupled to each other.
The bias magnetic field applying layers disclosed in JP8-315325A each include a magnetic separation layer and two magnetic field applying layers disposed to sandwich the magnetic separation layer. The two magnetic field applying layers are magnetized in opposite directions through the use of a difference in coercivity between the two layers.
The bias magnetic field applying layers disclosed in JP8-050709A are each formed of a stack of two hard magnetic layers having different coercivities.
Conventionally, in thin-film magnetic heads, there are cases in which the state of magnetization in the bias magnetic field applying layers or in the two shield layers disposed on the top and bottom of the MR element is changed by various factors, and as a result, there occurs a change in the bias magnetic field applied to the free layer to cause a sudden variation in the output signals of the read head. Factors that cause a change in the state of magnetization in the bias magnetic field applying layers or in the shield layers include, for example, a variation in an external magnetic field, stress generated in the bias magnetic field applying layers or in the shield layers due to a collision between the thin-film magnetic head and the recording medium, and stress generated in the bias magnetic field applying layers or in the shield layers due to heat generated when writing is performed by the write head.
The variation in the output signals of the read head mentioned above impairs the reliability of the thin-film magnetic head, and therefore it needs to be suppressed. One of effective measures against the variation in the output signals of the read head is to allow the bias magnetic field applying layers to generate a stable bias magnetic field that resists being changed by a variation in the external magnetic field and the generation of stress.
To allow the bias magnetic field applying layers to generate a stable bias magnetic field, it is effective to increase the thickness of the bias magnetic field applying layers to thereby increase the magnitude of the bias magnetic field, or to increase the anisotropic energy of the bias magnetic field applying layers to thereby increase the coercivity and the squareness ratio of the bias magnetic field applying layers.
However, an increase in thickness of the bias magnetic field applying layers leads to an increase in the read gap length or the distance between the two shield layers, and this hinders an increase in linear recording density of a magnetic disk drive.
In the bias magnetic field applying layers each formed of a hard magnetic layer, the hard magnetic layer has a magnetic anisotropy in the in-plane direction. It is difficult to increase the anisotropic energy of the bias magnetic field applying layers of this type, because of the reason described below. In the bias magnetic field applying layers each formed of a hard magnetic layer, anisotropic energy is obtained mainly through the use of magnetocrystalline anisotropy. However, in a case where the bias magnetic field applying layers are formed by an ordinary method such as sputtering, it is very difficult to align the magnetocrystalline anisotropy across the entire bias magnetic field applying layers. It is therefore difficult to increase the anisotropic energy of the bias magnetic field applying layers of this type.
In the bias magnetic field applying layers each formed of a stack of a ferromagnetic layer and an antiferromagnetic layer, as taught in JP9-180134A, if the thickness of the ferromagnetic layer is increased to obtain a bias magnetic field of greater magnitude, the magnitude of the exchange coupling magnetic field produced by the ferromagnetic layer and the antiferromagnetic layer is reduced and the bias magnetic field thereby becomes variable.
In the bias magnetic field applying layers each formed of a stack of three or more layers including a ferromagnetic layer, an antiferromagnetic layer and a ferromagnetic layer as disclosed in JP9-180134A, the proportion of the thickness of the antiferromagnetic layer in the total thickness of each bias magnetic field applying layer is relatively high, and therefore it is difficult to increase the magnitude of the bias magnetic field without increasing the total thickness of each bias magnetic field applying layer.
According to the bias magnetic field applying layers disclosed in JP8-315325A, the two magnetic field applying layers are simply magnetized in opposite directions through the use of the difference in coercivity between the two layers, and therefore it is not possible to obtain an effect of ensuring stability of the bias magnetic field generated from the bias magnetic field applying layers. Likewise, the bias magnetic field applying layers disclosed in JP8-050709A cannot provide the effect of ensuring stability of the bias magnetic field generated therefrom.